1. Field of the Invention
The present invention relates to a plasma spray deposition method for producing a material deposit onto a substrate. The plasma deposition method may be used either to produce a protective coating of the substrate or to form a near net shape body on this substrate. More particularly, the present invention relates to a plasma spray deposition method in which the material to be deposited is supplied in the form of a suspension including solid particles of the material dispersed into a liquid or semi-liquid carrier substance.
The present invention further relates to a method using the same technique for producing a powder of a given material from a suspension of that material.
2. Brief Description of the Prior Art
Plasma deposition of particulate materials, in particular ceramic and metallic powders, has been known and used on an industrial scale since the late 60's and early 70's.
However, the conventional plasma deposition methods using particulate materials such as ceramic and metallic powders have many drawbacks essentially related to the production of these powders. Indeed, powders having a high added value are generally chemically synthesized using precipitation or co-precipitation processes. Obtention of powders having adequate particle size distribution from these precipitates require many lengthy steps likely to introduce impurities in the powders. Obviously, these impurities degrade the properties of the powders.
For example, to produce hydroxyapatite (HAP) which is a bioceramic material having a chemical composition similar to that of human hard tissues (bones and teeth), calcium hydroxide is reacted with phosphoric acid to obtain a gel precipitate which is transformed into powder by means of the following steps:
drying at 180.degree. C.; PA1 calcination at 800.degree. C.; PA1 sintering at 1150.degree. C.; PA1 crushing; and PA1 screening. PA1 producing a plasma discharge; PA1 providing a suspension of a material to be deposited, this suspension comprising small solid particles of that material dispersed into a liquid or semi-liquid carrier substance; PA1 atomizing the suspension into a stream of fine droplets and injecting the stream of fine droplets within the plasma discharge; and PA1 by means of the plasma discharge, (a) vaporizing the carrier substance, (b) agglomerating the small particles into at least partially melted drops, (c) accelerating these drops, and (d) projecting the accelerated drops onto the substrate to form the material deposit. PA1 producing a plasma discharge; PA1 providing a suspension of that given material, this suspension comprising small solid particles of the material dispersed into a liquid or semi-liquid carrier substance; PA1 atomizing the suspension into a stream of fine droplets and injecting that stream of fine droplets within the plasma discharge; PA1 by means of the plasma discharge, vaporizing the carrier substance and agglomerating the small particles into at least partially melted drops; and PA1 solidifying these drops in-flight to thereby form a powder of the given material. PA1 an inner conduit member having a first end and a second discharge end, and being supplied with the suspension at the first end thereof; and PA1 an outer conduit member surrounding the inner conduit member to define an annular chamber between the inner and outer conduit members, the outer conduit member having a first end and a second end adjacent the second discharge end of the inner conduit member, and the annular chamber being supplied with a pressurized atomizing gas at the first end of the outer conduit member.
The product obtained from the screening step is a powder that can be deposited by known plasma deposition methods, usually involving a carrier gas to carry the powder to the plasma discharge.
Since HAP is a material used for coating implants, impurities is likely to cause biocompatibility problems.
Another important drawback of the plasma deposition of particulate materials injected in the plasma discharge by means of a carrier gas is the possible partial decomposition of the powder under the influence of the high temperature of the plasma discharge. The degree of decomposition depends on the plasma composition and the contact time.
In an attempt to overcome these drawbacks, it has been proposed to replace the powder by an aqueous solution of the material to be deposited (U.S. Pat. No. 5,032,568 granted to Lau et al. on Jul. 16, 1991). In this patent, metal salts are dissolved in water and the resulting aqueous solution is atomized and then injected into an inductively coupled radio-frequency plasma torch where it is vaporized to form on a target surface a film comprising a mixed oxide of the dissolved metal ions.
Applicability of the method of Lau et al. is limited since many materials to be deposited on a substrate cannot be dissolved in water or other liquid. Another drawback of the method of Lau et al. is that vaporisation of the dissolved material being deposited may lead to changes in the properties of the deposit. Finally, vapour deposition is very slow.